A "rocket" immunoelectrophoretic method was applied to quantitation of H. influenzae type b (Hib) PS in Hib conjugate alone and in a combined vaccines with DTP. Three types of Hib-protein conjugate vaccines were prepared by manufacturers by different methods, using different protein carrier, PS size, nature of linkage, and ratio of protein and PS. Furthermore, two manufacturers prepared DTP-Hib combined vaccine. With the optimum concentration of Hib antiserum (rabbit Hib antiserum, 20ul/ml agarose gel), linear curves were obtained with Hib PS-protein conjugate at concentrations of 2 to 10 ug Hib PS/ml. For two manufacturers conjugate vaccine samples resulted in clear, straight migrating rockets. In contrast, another vaccine showed skewed rockets with open ended at the tip. Different experimental parameters will be used to find the optimum condition for IEP. Quantitative determination of Hib conjugate could be analyzed accurately combined with DTP. A nephelometric method for quantitation of the Hib PS content of Hib conjugate and DTP-Hib conjugate combined vaccines has also been examined. More studies are required to obtain the optimum conditions.